Fuel Cell

ABSTRACT

Fuel cells and methods of operating fuel cells are disclosed. In one aspect, the invention features a fuel source for a fuel cell including a housing having an outlet, a structure having a portion in the housing, the structure defining a cavity and having a surface defining an opening in fluid communication with the cavity, and a fuel in the housing. The fuel is in fluid communication with the outlet through the opening and the cavity of the structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims priority toU.S. Ser. No. 10/779,502, filed on Feb. 13, 2004, which is herebyincorporated by reference.

FIELD OF THE INVENTION

The invention relates to fuel cells and methods of operating the cells.

BACKGROUND

A fuel cell is a device capable of providing electrical energy from anelectrochemical reaction, typically between two or more reactants.Generally, a fuel cell includes two electrodes, called an anode and acathode, and a solid electrolyte disposed between the electrodes. Theanode contains an anode catalyst, and the cathode contains a cathodecatalyst. The electrolyte, such as an membrane electrolyte, is typicallyionically conducting but electronically non-conducting. The electrodesand solid electrolyte can be disposed between two gas diffusion layers(GDLs).

During operation of the fuel cell, the reactants are introduced to theappropriate electrodes. At the anode, the reactant(s) (the anodereactant(s)) interacts with the anode catalyst and forms reactionintermediates, such as ions and electrons. The ionic reactionintermediates can flow from the anode, through the electrolyte, and tothe cathode. The electrons, however, flow from the anode to the cathodethrough an external load electrically connecting the anode and thecathode. As electrons flow through the external load, electrical energyis provided. At the cathode, the cathode catalyst interacts with theother reactant(s) (the cathode reactant(s)), the intermediates formed atthe anode, and the electrons to complete the fuel cell reaction.

For example, in one type of fuel cell, sometimes called a directmethanol fuel cell (DMFC), the anode reactants include methanol andwater, and the cathode reactant includes oxygen (e.g., from air). At theanode, methanol is oxidized; and at the cathode, oxygen is reduced:CH₃OH+H₂O→CO₂+6H⁺+6e⁻  (1)3/2O₂+6H⁺6e⁻→3H₂O   (2)CH₃OH+3/2O₂→CO₂+2H₂O   (3)As shown in Equation (1), oxidation of methanol produces carbon dioxide,protons, and electrons. The protons flow from the anode, through theelectrolyte, and to the cathode. The electrons flow from the anode tothe cathode through an external load, thereby providing electricalenergy. At the cathode, the protons and the electrons react with oxygento form water (Equation 2). Equation 3 shows the overall fuel cellreaction.

SUMMARY

The invention relates to fuel cells and methods of operating the cells.

In one aspect, the invention features a fuel cell system, such as a DMFCsystem, having a fuel source including vapor transmission element(s) anda fuel (such as a fuel gel) capable of emitting gas phase fuel. Thevapor transmission element(s) is capable of providing a high surfacearea, relatively constant interface with the fuel, thereby providingrelatively constant delivery of fuel. Furthermore, the high surface areainterface, combined with delivery of fuel in the gas phase, enhances thegeometric and orientational versatility of the fuel source. The fuelsource need not, for example, match the area footprint of the fuel cell.Vapor phase fuel delivery can also reduce migration of the fuel from theanode to the cathode (e.g., methanol crossover), which can causeparasitic loss (and reduced runtime) and mixed potentials at the cathode(and reduced output power.) What is more, the fuel source is capable ofproviding these features in a low volume design with good protectionagainst leaks.

In some embodiments, the fuel cell system includes one or more gasmovers that actively regulate flow of the gas phase fuel and furtherenhance the performance of the system, e.g., the orientationalversatility of the fuel source. Alternatively or in addition, the fuelcell system can include one or more restrictive mechanisms that regulateflow of the gas phase fuel.

In one aspect, the invention features a fuel source for a fuel cell,including a housing having an outlet; a structure having a portion inthe housing, the structure defining a cavity and having a surfacedefining an opening in fluid communication with the cavity; and a fuelin the housing, the fuel being in fluid communication with the outletthrough the opening and the cavity of the structure.

Embodiments may include one or more of the following features. Thestructure includes an elongated tube and the cavity is a lumen of thetube. The housing has an inlet, and the fuel source further includes adiffusion tube in fluid communication with the inlet. The housingincludes a fire-retardant material. The housing is configured to engagewith a fuel cell system. The opening is configured to reduce flow ofnon-gaseous fuel through the opening. The fuel source includes aplurality of structures in the housing. The structure includes ahydrophobic material. The fuel source further includes a diffusion tubein fluid communication with the outlet and/or a valve (such as a slitvalve) capable of selectively reducing fluid flow through the diffusiontube. The fuel source further includes a carbon dioxide getter in fluidcommunication with the fuel. The fuel includes a gel and/or a liquid.The fuel includes an alcohol, such as methanol.

In another aspect, the invention features a fuel source, including ahousing having an outlet; a plurality of elongated tubes having portionsin the housing, each tube defining a lumen and having a surface defininga plurality of openings; and a fuel comprising a gel in the housing, thefuel being in fluid communication with the outlet through the openingsand the lumens of the tubes, wherein the fuel source is configured to bein fluid communication with a fuel system. The fuel can includemethanol.

In another aspect, the invention features a fuel cell system, includinga fuel cell; a fuel source in fluid communication with the fuel cell,the fuel source comprising a fuel comprising an alcohol; and a gas moverbetween the fuel cell and the fuel source along a fluid flow path.

Embodiments may include one or more of the following features. The fuelsource includes a housing having an outlet, and the gas mover (such as afan) is between the outlet and the fuel cell along the fluid flow path.The fuel source includes a housing having an inlet, and the gas mover isbetween the inlet and the fuel cell along the fluid flow path. The fuelcell system further includes a diffusion tube between the fuel cell andthe fuel source along the fluid flow path. The fuel cell system furtherincludes a valve between the fuel cell and the fuel source along thefluid flow path. The fuel source includes a gel.

The fuel source can include a housing having an outlet, a structure inthe housing, the structure defining a cavity and having a surfacedefining an opening in fluid communication with the cavity; and a fuelin the housing, the fuel being in fluid communication with the outletthrough the opening and the cavity of the structure.

The fuel source can include a housing having an outlet; a plurality ofelongated tubes in the housing, each tube defining a lumen and having asurface defining a plurality of openings in fluid communication with thelumen; and a fuel comprising a gel in the housing, the fuel being influid communication with the outlet through the openings and the lumensof the tubes.

In another aspect, the invention features a method of operating a fuelcell system, including passing a fuel gas through an opening and acavity of a structure having a portion in a fuel source; and contactingthe fuel gas to an anode of a fuel cell.

Embodiments may include one or more of the following features. Themethod includes passing the fuel gas through a plurality of openings anda plurality of cavities of a plurality of structures having portions inthe fuel source. The structures include elongated tubes. The methodfurther includes passing the fuel gas through a diffusion tube;restricting the fuel gas with a valve; fanning the fuel gas from thefuel source to the fuel cell; and/or fanning a gas from an outlet of thefuel cell system to the fuel source, and contacting the gas to a fuel.The fuel source includes a liquid fuel or a gel fuel, such as oneincluding methanol.

In another aspect, the invention features a method of operating a fuelcell system, including fanning a fuel gas from a fuel source to a fuelcell, the fuel source comprising an alcohol; and contacting the fuel gasto an anode of the fuel cell.

Embodiments may include one or more of the following features. Themethod further includes passing the fuel gas through a diffusion tubeand/or reducing the flow of the fuel gas. The fuel source includes aliquid fuel or a gel fuel.

In another aspect, the invention features a method of operating a fuelcell system, including fanning a gas from an outlet of the fuel cellsystem to a fuel source; and contacting the gas with a fuel in the fuelsource, the fuel comprising an alcohol.

The method can further include passing the gas through a diffusion tube,contacting the gas to a desiccant or a carbon dioxide getter, and/orreducing the flow of the gas. The fuel source can include a liquid fuelor a gel fuel.

In another aspect, the invention features a method of operating a fuelcell system, including passing a fuel gas through a plurality ofopenings and a plurality of cavities of a plurality of structures havingportions in a fuel source; fanning the fuel gas from the fuel source toa fuel cell; and contacting the fuel gas to an anode of the fuel cell.

Embodiments may include one or more of the following features. Themethod further includes passing the fuel gas through a diffusion tube;reducing the flow of the fuel gas; fanning an outlet gas from the fuelcell to the fuel source, and contacting the outlet gas to a fuel in thefuel source; passing the outlet gas through a diffusion tube; and/orreducing the flow of the outlet gas. The fuel source includes a liquidfuel or a gel fuel.

Other aspects, features, and advantages of the invention will beapparent from the drawing, description, and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of a fuel cell system.

FIG. 2 is an illustration of an embodiment of a fuel source.

FIG. 3 is a schematic diagram of an embodiment of a fuel source.

FIG. 4 is a schematic diagram of an embodiment of a fuel source.

FIG. 5 is an illustration of an embodiment of a fuel source.

FIGS. 6A, 6B, and 6C are illustrations of gas diffusers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a fuel cell system 20, such as, a direct methanolfuel cell (DMFC) system, is shown. Fuel cell system 20 includes a fuelcell stack 22; a fuel source 24 (e.g., a cartridge containing methanol)in fluid communication with the fuel cell stack; two gas movers 26 and28 in fluid communication with the fuel cell stack and the fuel source;and an air mover 30. For illustrative purposes, fuel cell stack 22 isshown having one fuel cell 32 (described below), but in otherembodiments, the fuel cell stack includes a plurality of fuel cells 32,e.g., arranged in series or in parallel. Briefly, fuel cell 32 includesan anode 34, a cathode 36, and an electrolyte 38 between the anode andthe cathode. Fuel cell 32 further includes a gas diffusion layer (GDL)40 and 42 disposed on each side of the electrolyte 38, anode 34, andcathode 36 assembly. Air mover 30 is arranged to facilitate flow of acathode reactant (e.g., air) to cathode 36, and flow of cathodeproduct(s) (e.g., water) from the cathode. Gas movers 26 and 28 arearranged to facilitate flow of anode reactant(s) (e.g., methanol andwater) from fuel source 24 to an anode inlet 54 and, flow of anodeproduct(s) (e.g., carbon dioxide) away from an anode outlet 56 (asshown, to the fuel source).

Referring to FIG. 2, fuel source 24 is configured to deliver vapor phasefuel to fuel cell stack 22. As shown, fuel source 24 is in the form of aprismatic cartridge including a housing 44 having an inlet 47 and anoutlet 53. Within housing 44, fuel source 24 includes a fuel 48 and oneor more (as shown, four) vapor transmission elements 46 in the fuel(e.g., surrounded by the fuel). In some embodiments, a vaportransmission element 46 is an elongated structure having an outersurface that defines a cavity 50 and is perforated with one or moreopenings 52. Cavity 50 and opening(s) 52 are in fluid communication withinlet 47 and outlet 53. A vapor transmission element can be, forexample, an open-ended, perforated tubular structure defining a lumen,and in other embodiments, the vapor transmission element can have anycross-sectional configuration (e.g., triangular, square, rectangular, orregularly or irregularly polygonal having 3, 4, 5, 6, 7, 8 or moresides). Vapor transmission elements 46 can extend substantially anentire length of housing 44 or a portion thereof. The cross-sectionalwidth or diameter of vapor transmission elements can be from about 0.01micron to about 10 mm (such as from about 1 mm to about 5 mm, e.g.,about 1 mm). Fuel source 24 can include vapor transmission elements 46of the same size and configuration, or of different sizes and/orconfigurations. Vapor transmission elements 46 can be formed of amaterial, such as polypropylene, that is stable to long-term exposure tofuel 48. In some cases, such as in a DMFC system, the material used toform vapor transmission elements 46 is hydrophobic to reduce thelikelihood of water condensation, thereby keeping water in the vaporphase.

Openings 52 are sized to allow gaseous fuel to pass through the openingswhile preventing non-gaseous fuel (e.g., liquid fuel) to pass, e.g., dueto surface tension. Openings 52 can be, for example, circular, oval,regularly or irregularly polygonal having 3, 4, 5, 6, 7, 8 or moresides. In some cases, openings 52 have an average width or diameter fromabout 0.001 mm to about 5 mm; for example, the average width or diametercan be greater than or equal to about 0.05 mm, 0.5 mm, 1 mm, or 3 mm,and/or less than or equal to about 5 mm, 3 mm, 1 mm, 0.5 mm, or 0.05 mm.Openings 52 can be arranged in a regular pattern or randomly on vaportransmission elements 46.

Fuel source 24 can include any number or arrangement of vaportransmission elements 46. Vapor transmission elements 46 can be arrangedirregularly or regularly, such as in an array of rows and columns. Vaportransmission element(s) 46 can contact each other and/or an interiorsurface of housing 44. In some embodiments, vapor transmission elements46 are spaced from each other and/or housing 44. For example, vaportransmission elements 46 can be maintained in predetermined positionsusing a plastic wire frame or scaffold. The frame or scaffold also allowfuel source 24 to be loaded with fuel 48 while maintaining the positionsof vapor transmission elements 46. If the frame or scaffold is removedafter fuel loading, vapor transmission elements 46 may move and contacteach other as fuel 48 is consumed during use.

Fuel 48 is capable of providing fuel in gaseous form to fuel cell stack22. Fuel 48, such as one including an alcohol (e.g., methanol and/orethanol) or a hydrocarbon, can be in the form of a liquid or a gelhaving a vapor pressure sufficient to provide gaseous fuel to stack 22.A fuel gel is a viscous material (e.g., from about 0.05 to about 200,000centipoises) capable of emitting a pure and high concentration ofgas-phase fuel molecules. The viscosity can be, for example, greaterthan or equal to about 10,000, 25,000, 50,000, 100,000, or 150,000centipoises; and/or less than or equal to about 200,000, 150,000,100,000, 50,000, or 25,000 centipoises. An example of a fuel gelcomposition includes a fuel (e.g., methanol); a diluent (e.g., deionizedwater); a thickener (e.g., Carbopol EZ-3, an acidic,hydrophobically-modified, cross-linked polyacrylate powder); and aneutralizing agent (e.g., tri-isopropanolamine). Other fuel gels aredescribed in literature from Noveon that describe examples of the use ofCarbopol rheology modifiers (manufactured by BF Goodrich); andexemplified by cooking fuels (e.g., available from Sterno, andformulation examples listed by Noveon). Fuel gels with sufficiently highviscosity can be constrained in housing 44, for example, by usingplastic grids (not shown). As shown in FIG. 2, fuel 48 is constrained todefine two end spaces 51, which provide two interfaces from which fuelvapor can be emitted. During use, gas-phase fuel molecules are capableof flowing from fuel 48, through openings 52 and cavities 50 of vaportransmission elements, through outlet 53, and to anode 34 of fuel cell32.

The use of vapor transmission elements 46, in combination with a fuelcapable of providing gas phase fuel, can enhance delivery of the fuel infuel cell system 20 and its performance. Vapor transmission elements 46can provide a high surface area interface with fuel 48 within a fuelsource of any geometry, without increasing the footprint of the fuelsource. For example, in a 4×8×1 cm prismatic fuel cartridge (volume of32 cc), the surface area of the large area footprint of the cartridge is32 cm² for certain designs of fuel cartridges. In comparison, using fourvapor transmission elements (e.g., 2 mm diameter, slightly less than 8cm length, 50% permeable) yields two 4×1 cm rectangular interfaces (8cm² total from end spaces 51) plus the area provided by the vaportransmission elements (slightly less than 2.5 cm² each) to totalslightly less than 18 cm². For ten vapor transmission elements, thetotal surface area is slightly less than 33 cm² (slightly less than 25cm² for the elements plus 8 cm² for the two interfaces) at a volume costof about 2.5 cc (less than 10% of the fuel cartridge volume).Furthermore, by providing a high surface area interface with fuel 48,without necessarily increasing the cross-sectional area of outlet 53,vapor transmission elements 46 enhances the versatility of fuel source24, e.g., in terms of its geometry and/or orientation with respect tofuel cell stack 22. Fuel source 24 need not assume the same areafootprint of fuel cell 32 (i.e., substantially match the active area ofanode 34) for good performance. Also, since vapor transmission elements46 can contact fuel 48, the interface between the vapor transmissionelements 46 and the fuel can remain relatively constant, e.g., the fueldoes not recede from the vapor transmission elements. As a result, asfuel 48 is consumed during use, fuel source 24 is capable of maintaininga constant fuel delivery rate, thereby allowing fuel cell stack 22 toprovide a constant power output. Moreover, fuel source 24 is capable ofproviding the above features in a relatively simple (e.g., no movingparts), low volume design with good protection against leaks,particularly when fuel 48 includes a gel.

What is more, the performance of fuel system 20 can be further enhancedby using either or both gas movers 26 and 28. Gas movers 26 and 28regulate flow of gas to and from fuel cell stack 22, and further allowfuel source 24 more versatility in its orientation with fuel cell stack22. Gas mover 26 is positioned in a gas flow path between outlet 48 offuel source 24 and anode inlet 54 of fuel cell stack 22. Gas mover 26 iscapable of facilitating (e.g., drawing) flow of gas phase fuel fromvapor transmission elements 46 to fuel cell 32. Gas mover 28 ispositioned in a gas flow path between anode outlet 56 of fuel cell stack22 and inlet 46 of fuel source 24. Gas mover 28 is capable offacilitating flow of anode outlet gases (e.g., unreacted fuel, carbondioxide, and water) from fuel cell 32 to fuel source 24, e.g., torecycle water. Gas movers 26 and 28 can be, for example, a fan, such asone having a DC motor (available from Kot'l JinLong Machinery, Wenzhou,China PR) and an impeller. In other embodiments, gas movers 26 and 28can be a diaphragm pump and its variations, or a peristaltic pump.Examples of pumps are described in U.S. Pat. No. 6,274,261; WO 00/36696;WO 01/97317; WO 01/97318; WO 01/97319; and WO 02/31906, all of which arehereby incorporated by reference in their entirety. Gas mover(s) 26and/or 28 can be an integrated component of fuel cell stack 22, e.g.,located in or on the stack, or the gas mover(s) can be a component offuel source 24.

In some embodiments, fuel cell system 20 includes one or morerestrictive mechanisms that partially or completely reduce (e.g.,isolate) gas flow from and to fuel source 24 when the fuel cell systemis not in use. Restricting flow of gas to and from fuel source 24 canreduce degradation of fuel 48 (e.g., oxidation by air) and unnecessaryevaporation of fuel. Examples of restrictive mechanisms include apressure-sensitive valve, such as a slit valve made from a polymermembrane, or a pop-up valve. The pressure-sensitive valve canautomatically open when there is a pressure differential (e.g., when gasmover(s) 26 and/or 28 is activated), and close when the pressuredifferential is eliminated (e.g., the gas mover(s) is deactivated).Pressure-sensitive valves are described, for example, in U.S. Ser. No.10/236,126, filed Sep. 6, 2002. Other examples of restrictive mechanismsinclude a gravity-driven flap, an electromechanical valve, or amechanical valve, such as a manually operated latch or valve. Therestrictive mechanisms can extend across the cross section of inlet 47and/or outlet 53 of fuel source 24, and/or across any inlet(s) and/oroutlet(s) of fuel cell stack 22, in any combination.

Alternatively or in addition, referring to FIGS. 3 and 4, therestrictive mechanism can include a diffusion tube 49 in fluidcommunication with inlet 47 and/or outlet 48 of fuel source 24, and/oracross any inlet(s) and/or outlet(s) of fuel cell stack 22, in anycombination. A diffusion tube is a structure having a sufficient longcavity or lumen such that, absent a driving force (e.g., from a gasmover), gas diffusion through the tube is sufficiently low to provide aneffective valve. In some embodiments, the length of the diffusion tubeis greater than about half the length of the cartridge to the fulllength of the cartridge. More than one diffusion tube can be used, forexample, by providing a manifold external to the cartridge on bothsides, and multiple inlets and outlets. Diffusion tubes are alsodescribed in U.S. Ser. No. 09/400,020, filed Sep. 21, 1999, and U.S.Pat. Nos. 5,560,999 and 5,721,064. In some embodiments, the diffusiontube(s) includes one or more of the restrictive mechanisms (e.g., slitvalve or flap valve) extending across the cross section of the tube(s).The restrictive mechanism(s) can shorten the length of the diffusiontube(s) and/or enhance isolation of fuel source 24. The diffusiontube(s) can be a component of fuel source 24 (e.g., integrally formedwithin or outside housing 44) or an integrated component of fuel cellstack 32. The restrictive mechanisms described above can be used in fuelcell systems with or without gas movers 26 and 28.

Air mover 30 is configured to facilitate flow of cathode reactant(s)(e.g., oxygen) to cathode 36 and flow of cathode product(s) (e.g.,water) from the cathode. Air mover 30 can be the same as gas mover 26 or28, e.g., a fan.

Referring again to FIG. 1, an example of fuel cell 32 will now bedescribed. Fuel cell 32 includes electrolyte 38, anode 34 bonded on afirst side of the electrolyte, and cathode 36 bonded on a second side ofthe electrolyte. Electrolyte 38, anode 34, and cathode 36 are disposedbetween gas diffusion layers (GDLs) 40 and 42.

Electrolyte 38 should be capable of allowing ions to flow therethroughwhile providing a substantial resistance to the flow of electrons. Insome embodiments, electrolyte 38 is a solid polymer (e.g., a solidpolymer ion exchange membrane), such as a solid polymer proton exchangemembrane (e.g., a solid polymer containing sulfonic acid groups). Suchmembranes are commercially available from E.I. DuPont de Nemours Company(Wilmington, Del.) under the trademark NAFION. Alternatively,electrolyte 38 can also be prepared from the commercial productGORE-SELECT, available from W. L. Gore & Associates (Elkton, Md.).

Anode 34 can be formed of a material, such as a catalyst, capable ofinteracting with methanol and water to form carbon dioxide, protons andelectrons. Examples of such materials include, for example, platinum,platinum alloys (such as Pt—Ru, Pt—Mo, Pt—W, or Pt—Sn), platinumdispersed on carbon black. Anode 34 can further include an electrolyte,such as an ionomeric material, e.g., NAFION, that allows the anode toconduct protons. Alternatively, a suspension is applied to the surfacesof gas diffusion layers (described below) that face solid electrolyte38, and the suspension is then dried. The method of preparing anode 34may further include the use of pressure and temperature to achievebonding.

Cathode 36 can be formed of a material, such as a catalyst, capable ofinteracting with oxygen, electrons and protons to form water. Examplesof such materials include, for example, platinum, platinum alloys (suchas Pt—Co, Pt—Cr, or Pt—Fe) and noble metals dispersed on carbon black.Cathode 36 can further include an electrolyte, such as an ionomericmaterial, e.g., NAFION, that allows the cathode to conduct protons.Cathode 36 can be prepared as described above with respect to anode 34.

Gas diffusion layers (GDLs) 40 and 42 can be formed of a material thatis both gas and liquid permeable. Suitable GDLs are available fromvarious companies such as Etek in Natick, Mass., SGL in Valencia,Calif., and Zoltek in St. Louis, Mo. GDLs 40 and 42 can be electricallyconductive so that electrons can flow from anode 34 to an anode flowfield plate (not shown) and from a cathode flow field plate (not shown)to cathode 36.

Other embodiments of direct methanol fuel cells and fuel cell systems,including methods of use, are described, for example, in “Fuel CellSystems Explained”, J. Laraminie, A. Dicks, Wiley, N.Y., 2000; “DirectMethanol Fuel Cells: From a Twentieth Century Electrochemist's Dream toa Twenty-first Century Emerging Technology”, C. Lamy, J. Leger, S.Srinivasan, Modern Aspects of Electrochemistry, No. 34, edited by J.Bockris et al., Kluwer Academic/Plenum Publishers, New York (2001) pp.53-118; and “Development of a Miniature Fuel Cell for PortableApplications”, S. R. Narayanan, T. I. Valdez and F. Clara, in DirectMethanol Fuel Cells, S. R. Narayanan, S. Gottesfeld and T. Zawodzinski,Editors, Electrochemical Society Proceedings, 2001-4 (2001) Pennington,N.J., all hereby incorporated by reference.

Other Embodiments

Housing 44 of fuel source 24 can be of any configuration capable ofengaging with and providing a fuel to a fuel cell or a fuel cell stack.For example, referring to FIG. 5, a fuel source 24′ includes an inlet47′ and an outlet 53′ configured to engage with, e.g., snap fit with orlock with, a corresponding inlet and an outlet of a fuel cell or fuelcell stack. Source 24′ can include any one or more of the restrictivemechanisms described above.

In some embodiments, housing 44 of fuel source 24 includes (e.g., formedfrom a composition including) a fire retardant (e.g., CN-2616, anitrogen-phosphorus intumescent compound available from Great LakesChemical) to reduce the flammability of the fuel source.

Anode inlet 54 can include one or more features to enhance evendistribution of fuel over the active area of anode 54. For example,referring to FIGS. 6A, 6B, and 6C, the fuel cell system can include oneor more gas diffusers 60, 62, and/or 64 for efficient vapor flow. Thegas diffusers can be, for example, a frustoconical member (FIG. 6A), aplate (FIG. 6B), or a screen (FIG. 6C) that engages with or is nearanode inlet 54. As gas exits anode inlet 54 and enters the anodechamber, the gas is dispersed over the anode chamber to reduce theoccurrence of areas of low fuel concentration relative to other areasacross the face of the MEA. The gas diffusers can be integral with thefuel source, the fuel cell, or the fuel cell stack. The gas diffuserscan be used on the cathode side of a fuel cell system. Other methods ofdispersing anode and/or cathode gases from a single inlet feed includesplitting the inlet into multiple ports, e.g., using a manifold.

In some embodiments, a fuel cell system includes only one gas mover,e.g., gas mover 26 or gas mover 28.

In some embodiments, the fuel cell systems described herein include oneor more emission control systems, such as those described in commonlyassigned U.S. Ser. No. 10/438,031, filed on May 14, 2003. Briefly, theemission control systems are capable of reducing the amount of unreactedfuel and products of partial oxidation (e.g., formaldehyde and/or formicacid) released into the environment. Embodiments of emission controlsystems, including their placements in the fuel cell system, aredescribed in U.S. Ser. No. 10/438,031.

Alternatively or in addition, the fuel cell system can include amaterial (e.g., a getter material) capable of reducing carbon dioxideemissions from fuel cell stack 22. The getter material, such as calciumoxide, can be located anywhere along a fluid flow path downstream ofoutlet 56. For example, fuel source 24 can contain the getter materialin housing 44.

In some embodiments, the vapor transmission elements do not include anyopenings. The vapor transmission elements can be formed of agas-permeable, liquid-impermeable material, such as a membrane ofexpanded polytetrafluoroethylene (PTFE), polypropylene, polystyrene, andpolyethylene, as described in U.S. Patent Application Publication US2003/0215686 A1, and Lim et al., Gas Permeable Membranes Composed ofCarboxylated Poly(vinyl chloride) and Polyurethane, Bull. Korean Chem.Soc. 1999, Vol. 20, No. 6, 672-676, hereby incorporated by reference.The gas-permeable, liquid-impermeable material can be used with a gelfuel and/or a liquid fuel. The vapor transmission elements formed of agas-permeable, liquid-impermeable material can be used with anyembodiment of fuel source or fuel cell system described herein.

EXAMPLE

The following illustrative example describes a method of making a fuelgel. The fuel gel includes about 75.0% by weight of methanol; about24.0% by weight of deionized water; about 0.5% by weight of CarbopolEZ-3; and about 0.5% by weight of tri-isopropanolamine.

The Carbopol EZ-3 thickener was added to the water without agitation andallowed to wet out for a few minutes. The methanol is then added, withagitation. Molten tri-isopropanolamine is then added with agitation.Other additives (such as a fire retardant, e.g., CN-2616) can also beadded after this step. After the addition of tri-isopropanolamine, aclear gel forms, which can be poured or pumped into a fuel cartridge,allowed to cool, and gel further.

All references, such as patent applications, publications, and patents,referred to herein are incorporated by reference in their entirety.

Other embodiments are in the claims.

1. A fuel source for a fuel cell, comprising: a housing having an outletfor a gaseous fuel; a non-gaseous fuel in the housing, the fuelcomprising an alcohol or a hydrocarbon; and a vapor transmission elementin the non-gaseous fuel and comprising a cavity and openings thatprevent the non-gaseous fuel from passing into the cavity while allowingthe non-gaseous fuel that converts to a gaseous fuel to pass into thecavity, wherein when the fuel source is in operation the non-gaseousfuel converts to gaseous fuel that passes through the openings into thecavity and then through the outlet of the housing.
 2. The fuel source ofclaim 1, wherein the vapor transmission element comprises an elongatedtube and the cavity is a lumen of the tube.
 3. The fuel source of claim1, wherein the openings are configured to reduce flow of non-gaseousfuel through the openings.
 4. The fuel source of claim 1, comprising aplurality of the vapor transmission elements in the non-gaseous fuel. 5.The fuel source of claim 1, wherein the vapor transmission elementcomprises a hydrophobic material.
 6. The fuel source of claim 1, whereinthe vapor transmission element is a diffusion tube.
 7. The fuel sourceof claim 6, further comprising a valve capable of selectively reducingthe passage of gaseous fuel from the vapor transmission element and outof the outlet.
 8. The fuel source of claim 7, wherein the valvecomprises a slit valve.
 9. The fuel source of claim 1, wherein thehousing further comprises an inlet for non-aqueous fuel, and the fuelsource further comprises a diffusion tube.
 10. The fuel source of claim1, wherein the non-aqueous fuel is a gel.
 11. The fuel source of claim1, wherein the non-aqueous fuel is a liquid.
 12. The fuel source ofclaim 1, wherein the fuel is an alcohol.
 13. The fuel source of claim12, wherein the fuel is methanol.
 14. The fuel source of claim 1,further comprising a carbon dioxide getter in fluid communication withthe fuel.
 15. The fuel source of claim 1, wherein the housing comprisesa fire-retardant material.
 16. The fuel source of claim 1, wherein thehousing is configured to engage with a fuel cell system.
 17. A fuelsource, comprising: a housing having an outlet for gaseous fuel; aplurality of elongated tubes in the housing, each tube defining a lumenand having a surface defining a plurality of openings; and a fuelcomprising a gel in the housing, the fuel being in communication withthe outlet through the openings and the lumens of the tubes, wherein thefuel source is configured to be in communication with a fuel system. 18.The fuel source of claim 17, wherein the fuel is methanol.
 19. A fuelsource for a fuel cell, comprising: a housing having an outlet; astructure in the housing, the structure defining a cavity and comprisingan opening and a gas-permeable, liquid-impermeable material; and a fuelin the housing, the fuel being in communication with the outlet throughthe opening and the cavity of the structure.
 20. The fuel source ofclaim 19, wherein the gas-permeable, liquid-impermeable materialcomprises polytetrafluoroethylene.
 21. The fuel source of claim 19,wherein the fuel comprises a liquid or a gel.
 22. The fuel source ofclaim 19, comprising a plurality of said structures in the housing. 23.The fuel cell system of claim 19, wherein the non-gaseous fuel ismethanol.